The present invention relates to programmable gain amplifiers and, more particularly, to a programmable gain amplifier having impedance networks enabling exponential gain and loss which may be implemented in bipolar and CMOS technologies.
Programmable gain amplifiers (PGAs) are used in many applications including digital cameras, camcorders and disk drives. Advances in integrated circuit design and manufacturing have enabled low cost, highly integrated, high performance image processing products such as the digital camera and camcorder. The market for smaller disk drives having reduced power consumption has increased the need for integrated read channel electronics on a single mixed-signal CMOS chip. Existing standard bipolar PGAs dissipate a substantial amount of power and exhibit linear gain. Since a PGA forms an important component of the read channel and helps to stabilize the voltage supplied to the detector and filter sections of the reach channel, it too must be integrated into a single mixed-signal CMOS chip. Previous attempts at developing a CMOS compatible PGA have resulted in a PGA design having the necessary gain control characteristics yet still dissipating substantial power.
An example of such, disclosed in U.S. Pat. No. 5,486,791, which is incorporated by reference herein, presents a PGA including a first and second gain element connected by an impedance selector which allows programmability of the gain of both gain elements. The impedance selector places impedance in the feedback path of the first gain element or the input path of the second gain element. This approach, however, dissipates a substantial amount of power and has a linear gain increase. Exponential gain to linear control voltage characteristic is preferred over linear gain for read channel automatic gain control loops to minimize variations in the output voltage. Thus, most disk drives require that the PGA exhibit exponential gain. Therefore, this approach is not suitable for read channel automatic gain control loops.
An approach having an exponential gain is disclosed in U.S. Pat. No. 6,049,252, which is incorporated by reference herein. The PGA provides sufficiently fine and linear gain control to provide an exponential output according to the logic levels of control signals. It includes a cascade connection of differential amplifiers and a current controller for controlling a product of corresponding emitter currents supplied to each of the differential amplifiers according to the logic levels of binary control signals. The current controller has a current mirror circuit in which the emitter of each bipolar transistor is connected to the ground through a MOS transistor having a relative on-resistance designed to be in inverse proportion to the relative emitter size of the bipolar transistor, so that the emitter current can be exactly controlled. Bipolar transistors having the same size or units of impedance are used in this design to implement the exponential gain. The use of these transistors, however, limits the range of the PGA. Furthermore, each transistor may not be linear. Moreover, even though this design may result in a PGA having the necessary gain control characteristics, it still dissipates substantial power. A significant percentage of this power is dissipated in generating the exponential current output for the linear voltage input. Although this approach, is implemented in bipolar technology, it is not easily implemented in CMOS as is explained in reference to a third approach.
This third approach disclosed in xe2x80x9cA Low-Power CMOS VGA for 50 Mb/s Disk Drive Read Channels,xe2x80x9d Ramesh Harjani, IEEE Transactions on Circuits and Systemsxe2x80x94II: Analog and Digital Signal Processing, Vol. 42, No. 6, June 1995, is a methodology for generating the desired exponential transfer characteristics intrinsically using only MOS devices within the variable gain amplifier structure. This approach appears to be a CMOS implementation of the second approach cited above. Since CMOS transistors have a square root relationship, however, it necessitates the implementation of duplicate circuitry to get rid of the square root relationship. As a result of multiplying two square root circuits to eliminate the square root relationship, linear gain exists. In addition, this implementation is very cumbersome to implement, the range is limited, and distortion exists.
Besides the desired exponential transfer characteristics, applications such as the digital camera require that a PGA operate under conditions where current is not continuously supplied. The conventional digital camera includes an image sensor, an analog front end (AFE), and a digital image processor. Most AFEs include three elements: a correlated double sampler (CDS), a programmable gain amplifier (PGA) and an analog to digital converter (ADC). In operation, a signal comes into the camera and is held. No continuous signal exists in such an application. The third approach presented above, however, requires that current be continuously applied to the PGA input in order to operate correctly.
Thus, there exists a need for a CMOS PGA having an exponential gain that operates given non-continuous current conditions.
To address the above-discussed deficiencies of programmable gain amplifiers (PGAs), the present invention teaches a PGA having an exponential gain including a gain element wherein the PGA may be implemented in either bipolar or CMOS technology.
One embodiment of the present invention includes a sampling impedance, a feedback impedance, and a gain element. The gain element having an input and an output. The input connects to the sampling impedance. The feedback impedance connects between the input and the output.       "LeftBracketingBar"                            V          0                +                  -                      V            0                          -                              V          i                +                  -                      V            i                          -              "RightBracketingBar"    =                              (                      1            +            X                    )                ⁢                  C          S                                      (                      1            -            X                    )                ⁢                  C          S                      =                  1        +        X                    1        -        X            
Another embodiment of the present invention includes a first and second sampling impedance, a first and second feedback impedance and a gain element. The gain element having an inverting and non-inverting input and an inverting and non-inverting output. The inverting input connects to the first sampling impedance. The non-inverting input connects to the second sampling impedance. The first feedback impedance connects between the inverting input and the non-inverting output. The second feedback impedance connects between the non-inverting input and the inverting output.
In another embodiment of the present invention includes a first and second sampling impedance, a first, second, third and fourth feedback impedance and a gain element. The gain element having an inverting and non-inverting input and an inverting and non-inverting output. The inverting input connects to the first sampling impedance. The non-inverting input connects to the second sampling impedance. The first feedback impedance connects between the inverting input and the non-inverting output. The second feedback impedance connects between the non-inverting input and the inverting output. The third feedback impedance connects between the inverting input and the inverting output. The fourth feedback impedance connects between the non-inverting input and the non-inverting output.
Advantages of this design include but are not limited to programmable gain amplifier implementable in bipolar or CMOS technology. This PGA has exponential gain characteristics without dissipating substantial power. Since the gain characteristic of the programmable gain amplifier in accordance with the present invention depends upon the impedance of a capacitor, the gain is more precise than others.